This invention relates to substrates covered with a protective coating. More particularly, it relates to improved primers for substrates adapted to receive a silicone resin top coat composition. Curing the top coat with heat forms an adherent protective, abrasion-resistant layer thereon.
Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering and lighter than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and marring results in impaired visibility and poor aesthetics, and oftentimes requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as LEXAN.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated. Acrylics, such as polymethylmethacrylate, are also widely used transparent plastics for glazing.
Attempts have been made to improve the abrasion resistance of transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. Misch, U.S. Pat. No. 3,708,225; Clark, U.S. Pat. Nos. 3,986,997, 3,986,497, and 4,027,073; Armbuster et al., U.S. Pat. No. 4,159,206; and Ubersax, U.S. Pat. No. 4,177,315 for example, describe such compositions. Improved such compositions are also described in commonly-assigned copending U.S. application Ser. No. 964,911, filed on Nov. 30, 1978; and in copending U.S. application Ser. No. 82,163, filed Oct. 5, 1979.
The application of silicone hard coat materials to LEXAN.RTM. and other thermoplastic materials, e.g., poly(methylmethacrylate) is pre-treatment dependent, i.e., it is good practice, if not essential, to prime the substrate. See, for example, Humphrey, U.S. Pat. No. 4,188,451, which uses as a primer, a UV-cured reaction product of a polyfunctional acrylic ester monomer and a silane containing a silicon-bonded unsaturated hydrocarbon radical, and the abovementioned Ser. No. 82,163, which discloses the use of a thermosetting acrylic resin. It is apparent that the current state of the art in primers requires a stoving (i.e., heating) step or a ultraviolet (uv) radiation step for proper performance. The contents of the abovementioned patents and applications are incorporated herein by reference.
It has now been discovered that thermoplastic acrylates can be laid down as primers by flowing, dipping and the like from a unique solvent system and, after simply air drying for 20-45 minutes, top coating with a thermally curable silica filled silicone will produce ultimately articles having a top layer with excellent adhesion, abrasion resistance and moisture resistance.